Simplify _calculate_fan_in_and_fan_out (#29370)
Summary:
The code checking `if dimensions == 2` is not needed
because the case of a 2D tensor (Linear) is already handled
by the statement:
`receptive_field_size = 1`
and this conditional:
`if tensor.dim() > 2:`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29370
Differential Revision: D18372987
Pulled By: albanD
fbshipit-source-id: fcb4dddbc76b9f4414c6d88c0aa2fb4435bf3385
